Conditional aggregation pattern in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the conditional aggregation pattern in SQL?

It is a way to calculate sums, counts, or other aggregates based on conditions inside a single query, using CASE statements inside aggregate functions.

beginner

How do you count rows that meet a condition using conditional aggregation?

Use COUNT(CASE WHEN condition THEN 1 END) or SUM(CASE WHEN condition THEN 1 ELSE 0 END) to count only rows that satisfy the condition.

intermediate

Why use conditional aggregation instead of multiple queries?

It combines multiple conditional counts or sums into one query, making it faster and easier to read, like counting different categories in one go.

beginner

Write a simple example of conditional aggregation to count male and female employees.

SELECT COUNT(CASE WHEN gender = 'M' THEN 1 END) AS male_count, COUNT(CASE WHEN gender = 'F' THEN 1 END) AS female_count FROM employees;

beginner

What happens if the CASE condition is false in conditional aggregation?

The CASE returns NULL or 0 (if ELSE 0 is used), so the aggregate function ignores or does not add that row to the count or sum.

Which SQL function is commonly used with CASE for conditional aggregation?

A SUM

B JOIN

C ORDER BY

D GROUP BY

SUM is often combined with CASE to add values conditionally in one query.

How do you count rows where a condition is true using conditional aggregation?

A WHERE condition

B COUNT(CASE WHEN condition THEN 1 END)

C GROUP BY condition

D ORDER BY condition

COUNT with CASE counts only rows where the condition is true.

What does this SQL snippet do? SUM(CASE WHEN status = 'active' THEN 1 ELSE 0 END)

A Sums all rows

B Counts inactive rows

C Counts active rows

D Filters rows

It sums 1 for each active row, effectively counting them.

Why is conditional aggregation useful?

A To combine multiple conditional counts or sums in one query

B To delete rows

C To create new tables

D To sort data

It helps summarize data by different conditions efficiently.

What will happen if CASE returns NULL inside an aggregate function like COUNT?

A The query fails

B NULL is counted as zero

C The row is counted anyway

D That row is ignored in the count

COUNT ignores NULL values, so those rows are not counted.

Explain how to use the conditional aggregation pattern to count different categories in one SQL query.

Think about how CASE can return values only when conditions are met.

Describe the difference between filtering rows with WHERE and counting conditionally with CASE inside aggregation.

Consider when rows are included or excluded in each method.

Practice

1. What does conditional aggregation in SQL allow you to do?
SUM(CASE WHEN condition THEN value ELSE 0 END)

easy

A. Delete rows that meet a condition

B. Join two tables based on a condition

C. Create a new table from existing data

D. Calculate sums or counts based on specific conditions within one query

Solution

1. Step 1: Understand conditional aggregation concept

   Conditional aggregation uses CASE inside aggregate functions like SUM or COUNT to selectively include values based on conditions.

2. Step 2: Identify what it achieves

   This lets you calculate sums or counts for different conditions in one query without multiple scans.

3. Final Answer:

   Calculate sums or counts based on specific conditions within one query -> Option D

4. Quick Check:

   Conditional aggregation = selective sums/counts [OK]

Hint: Think: aggregate + CASE filters rows inside one query [OK]

Common Mistakes:

• Confusing conditional aggregation with JOIN operations
• Thinking it creates or deletes tables
• Assuming it filters rows outside aggregation

2. Which of the following is the correct syntax using COUNT for counting only rows where status = 'active' using conditional aggregation?

easy

A. COUNT(CASE WHEN status = 'active' THEN 1 ELSE NULL END)

B. COUNT(CASE status = 'active' THEN 1 END)

C. SUM(CASE WHEN status = 'active' THEN 1 ELSE 0 END)

D. COUNT(IF status = 'active' THEN 1 ELSE 0 END)

Solution

1. Step 1: Recall correct CASE syntax inside COUNT

   The CASE expression must have WHEN condition THEN value ELSE NULL END for COUNT to count only matching rows.

2. Step 2: Check each option

   COUNT(CASE WHEN status = 'active' THEN 1 ELSE NULL END) uses correct CASE syntax with ELSE NULL, so COUNT counts only 'active' rows. COUNT(CASE status = 'active' THEN 1 END) misses WHEN keyword. SUM(CASE WHEN status = 'active' THEN 1 ELSE 0 END) uses SUM, not COUNT. COUNT(IF status = 'active' THEN 1 ELSE 0 END) uses invalid IF syntax.

3. Final Answer:

   COUNT(CASE WHEN status = 'active' THEN 1 ELSE NULL END) -> Option A

4. Quick Check:

   COUNT + CASE WHEN ... THEN 1 ELSE NULL END [OK]

Hint: Use ELSE NULL inside CASE for COUNT to skip rows [OK]

Common Mistakes:

• Omitting WHEN keyword in CASE
• Using SUM instead of COUNT for counting rows
• Using ELSE 0 which counts zeros as rows

3. Given the table sales with columns region and amount, what is the output of this query?

SELECT region, SUM(CASE WHEN amount > 100 THEN amount ELSE 0 END) AS big_sales FROM sales GROUP BY region;

medium

A. Sum of sales amounts greater than 100 per region

B. Count of sales with amount greater than 100 per region

C. Sum of all sales amounts per region

D. Sum of sales amounts less than or equal to 100 per region

Solution

1. Step 1: Analyze the CASE inside SUM

   The CASE returns amount if amount > 100, else 0. So only amounts above 100 contribute to the sum.

2. Step 2: Understand GROUP BY effect

   The query groups rows by region, summing only big sales (amount > 100) per region.

3. Final Answer:

   Sum of sales amounts greater than 100 per region -> Option A

4. Quick Check:

   SUM with CASE filters amounts > 100 [OK]

Hint: CASE filters values inside SUM to include only big sales [OK]

Common Mistakes:

• Thinking it sums all sales, ignoring condition
• Confusing SUM with COUNT
• Assuming it sums small sales instead

4. Identify the error in this SQL query using conditional aggregation:

SELECT department, COUNT(CASE WHEN salary > 50000 THEN 1 ELSE 0 END) AS high_earners FROM employees GROUP BY department;

medium

A. SUM should be used instead of COUNT

B. COUNT counts zeros, so it counts all rows, not just high earners

C. CASE syntax is invalid

D. Missing GROUP BY clause

Solution

1. Step 1: Understand COUNT behavior with CASE

   COUNT counts all non-NULL values. Here ELSE 0 returns zero, which is counted as a value, so all rows count.

2. Step 2: Identify correct fix

   To count only high earners, ELSE should be NULL so those rows are ignored by COUNT.

3. Final Answer:

   COUNT counts zeros, so it counts all rows, not just high earners -> Option B

4. Quick Check:

   COUNT counts non-NULL, zero is counted [OK]

Hint: Use ELSE NULL to exclude rows from COUNT [OK]

Common Mistakes:

• Using ELSE 0 causes wrong counts
• Confusing COUNT with SUM
• Ignoring that zero is counted by COUNT

5. You want to find the total sales and the count of sales above 1000 for each product in one query. Which SQL query correctly uses conditional aggregation?

hard

A. SELECT product, COUNT(amount) AS total_sales, SUM(CASE WHEN amount > 1000 THEN 1 ELSE 0 END) AS big_sales_count FROM sales GROUP BY product;

B. SELECT product, SUM(CASE WHEN amount > 1000 THEN amount ELSE 0 END) AS total_sales, COUNT(amount) AS big_sales_count FROM sales GROUP BY product;

C. SELECT product, SUM(amount) AS total_sales, SUM(CASE WHEN amount > 1000 THEN 1 ELSE 0 END) AS big_sales_count FROM sales GROUP BY product;

D. SELECT product, SUM(amount) AS total_sales, COUNT(CASE WHEN amount > 1000 THEN 1 ELSE 0 END) AS big_sales_count FROM sales GROUP BY product;

Solution

1. Step 1: Identify total sales calculation

   Total sales is sum of all amounts, so SUM(amount) is correct.

2. Step 2: Identify count of sales above 1000

   Counting sales above 1000 can be done by summing 1 for each qualifying row and 0 otherwise, so SUM(CASE WHEN amount > 1000 THEN 1 ELSE 0 END) works.

3. Step 3: Check each option

   SELECT product, SUM(amount) AS total_sales, COUNT(CASE WHEN amount > 1000 THEN 1 ELSE 0 END) AS big_sales_count FROM sales GROUP BY product; uses COUNT with ELSE 0 which counts all rows incorrectly; SELECT product, SUM(CASE WHEN amount > 1000 THEN amount ELSE 0 END) AS total_sales, COUNT(amount) AS big_sales_count FROM sales GROUP BY product; incorrectly sums only amounts above 1000 for total_sales; SELECT product, COUNT(amount) AS total_sales, SUM(CASE WHEN amount > 1000 THEN 1 ELSE 0 END) AS big_sales_count FROM sales GROUP BY product; swaps SUM and COUNT incorrectly; SELECT product, SUM(amount) AS total_sales, SUM(CASE WHEN amount > 1000 THEN 1 ELSE 0 END) AS big_sales_count FROM sales GROUP BY product; correctly sums total sales and counts big sales using SUM with CASE.

4. Final Answer:

   SELECT product, SUM(amount) AS total_sales, SUM(CASE WHEN amount > 1000 THEN 1 ELSE 0 END) AS big_sales_count FROM sales GROUP BY product; -> Option C

5. Quick Check:

   SUM for totals, SUM(CASE) for conditional counts [OK]

Hint: Use SUM(CASE) for conditional counts, SUM(column) for totals [OK]

Common Mistakes:

• Using COUNT with ELSE 0 which counts all rows
• Summing only filtered amounts for total sales
• Mixing COUNT and SUM incorrectly